A fuel cell (e.g., a polymer electrolyte fuel cell) is configured to supply a fuel gas such as hydrogen to the one face of a polymer electrolyte membrane possessing hydrogen ion conductivity, and to supply an oxidant gas such as oxygen to the other face, to cause an electrochemical reaction via the polymer electrolyte membrane, so as to electrically acquire reaction energy produced thereby.
Generally, a fuel cell is structured by stacking a plurality of cells, and fastening them under pressure with fastening members such as bolts. One cell is structured with a membrane electrode assembly (hereinafter, referred to as the MEA: Membrane-Electrode-Assembly) clamped by paired plate-like electrically conductive separators. One of the paired separators is an anode separator. The anode separator has fluid flow passages for supplying the fuel gas, which is formed on the face of the anode separator contacting on the MEA. The other one of the paired separators is a cathode separator. The cathode separator has fluid flow passages for supplying the oxidant gas, which is formed on the face of the cathode separator contacting on the MEA.
The MEA is structured with a polymer electrolyte membrane, and paired porous electrodes respectively disposed on opposite faces of the polymer electrolyte membrane. One of the paired porous electrodes is an anode electrode, and the other is a cathode electrode. The polymer electrolyte membrane and the porous electrodes are integrally joined with hot press or the like. The paired porous electrodes are each structured with a catalyst layer stacked on the polymer electrolyte membrane, and a gas diffusion layer stacked on the catalyst layer, the gas diffusion layer being porous and conductive.
When the fuel gas is introduced into the fluid flow passages of the anode separator and the oxidant gas is introduced into the fluid flow passages of the cathode separator, an electrochemical reaction takes place via the polymer electrolyte membrane. The fuel cell is structured such that the electric power generated by the electrochemical reaction is acquired externally via the separators.
In the fuel cell of such a structure, the porous electrodes are required to possess excellent conductivity, air permeability, water permeability, and corrosion resistance. Accordingly, the porous electrodes are made of, for example, graphite carbon, which possesses excellent conductivity, corrosion resistance, and water repellency. Such graphite carbon can be prepared, for example, as follows: turning the carbon fibers into a paper-form or weaving the carbon fibers, to thereby prepare a sheet possessing air permeability attributed to its structure; and thereafter, subjecting the sheet to heat treatment, to thereby improve its graphitization degree.
Further, the separators are required to possess excellent conductivity, gas impermeability, and corrosion resistance. Accordingly, as the material of the separators, for example, a conductive material made of a graphite base material, a metal base material or the like is used. When a graphite base material is used as the material of the separators, the fluid flow passages of the separators are generally formed by subjecting a mixture of graphite powder and resin to compression molding. Further, when a metal base material is used as the material of the separators, the fluid flow passages of the separators are generally formed by performing presswork of a thin plate treated to be conductive on the surface of a highly corrosion resistive material such as stainless steel, titanium or the like. It is noted that, a separator which is formed of a mixture of graphite powder and resin is disclosed, for example, in Patent Document 1 (International Publication No. WO 2002-035630). Patent Document 1 discloses a separator in which a plurality of flat graphite particles are exposed at the surface of the separator, and a resin lacking portion is formed between the plurality of flat graphite particles, for the purpose of reducing the contact resistance between the separator and the gas diffusion layer.
However, the separator using the graphite base material is low in strength, and therefore a reduction in thickness is difficult to be achieved. Therefore, it is disadvantageous in attaining a reduction in size and costs of the fuel cell. Further, the separator using metal base material is difficult to freely pattern the fluid flow passages.
As a solution for such issues, a fuel cell having the fluid flow passages disposed at a member other than the separator is disclosed in, for example, Patent Document 2 (Japanese Unexamined Patent Publication No. 2000-123850), Patent Document 3 (Japanese Unexamined Patent Publication No. 2002-203571), and Patent Document 4 (Japanese Unexamined Patent Publication No. 2006-339089).
FIG. 13 shows the structure of the fuel cell disclosed in Patent Document 2. The fuel cell of Patent Document 2 has a structure in which a catalyst layer 102, a first carbon sheet 103, a second carbon sheet 104, and a separator 105 are stacked in order on each face of a polymer electrolyte membrane 101. The second carbon sheet 104 is cut into a pattern shape of a fluid flow passage 106, to structure a gas diffusion layer with the first carbon sheet 103. That is, in the fuel cell of the Patent Document 2, the fluid flow passage 106 is secured by disposing the second carbon sheet 104 between the separator 105 and the first carbon sheet 103.
FIG. 14 shows the structure of the fuel cell disclosed in Patent Document 3. The fuel cell of Patent Document 3 has a structure in which a porous electrode 202 and a separator 203 are stacked in order on each face of a polymer electrolyte membrane 201. Each porous electrode 202 is structured by weaving carbon fibers of a low degree of crystal orientation and a great surface area into a sheet 205 made of carbon fibers of a high degree of crystal orientation and a small surface area, to form sidewall portions 206 of fluid flow passages 204. That is, in the fuel cell of Patent Document 3, each fluid flow passage 204 is formed at the porous electrode 202 serving as the gas diffusion layer. Further, in the fuel cell of Patent Document 3, the gas diffusion layer is structured to possess both the water repellency and water retentivity, by causing the carbon fibers structuring the bottom face of the fluid flow passage 204 and the carbon fibers structuring the side face of the fluid flow passage 204 to be different from each other in pore density.
FIG. 15A shows the structure of the fuel cell disclosed in Patent Document 4. The fuel cell of Patent Document 4 has a structure in which a catalyst layer 302, a gas diffusion layer 303, and a separator 304 are stacked in order on each face of a polymer electrolyte membrane 301. FIG. 15B is a cross-sectional view showing the structure of the gas diffusion layer 303. The gas diffusion layer 303 is structured by stacking a water-repellent layer 305 provided on the polymer electrolyte membrane 301 side, and a gas flow passage layer 307, the gas flow passage layer 307 having fluid flow passages 306 formed inside its body formed by a porous body. That is, in the fuel cell of Patent Document 4, the gas diffusion layer 303 is prepared as a two-layer structure made up of a water-repellent layer 305 and a gas flow passage layer 307, such that the fluid flow passages 306 are formed inside the gas diffusion layer 303.